Use of Polymeric Borate-Ester Electrolyte Additives to Stabilize the Interface and Enhance the Cycling Performance of Silicon/Graphite Composites in Lithium-Ion Batteries

In order to improve the energy density of Li-ion batteries, in this work, we investigate the effect of newly synthesized polymeric borate ester (PBE) additives on the electrochemical performance of the high-capacity silicon-graphite (Si-C) anode in half-cells coupled with the LiNi1/3Mn1/3Co1/3O2 (NMC532) cathode in full cells with a direct comparison to the conventional fluoroethylene carbonate (FEC) additive. We present a one-step route to prepare polymer additives having borate ester groups by reacting organic diols with boric acid in a simple condensation reaction. One polymer synthesized with diethylene glycol (PBE-DG) is studied extensively, and then its chemical structure is confirmed by 1H NMR and Fourier transform infrared spectroscopy (FTIR) spectroscopy. When used as an additive in carbonate electrolyte solutions, it shows excellent electrochemical stability and improved battery performance. It is shown that after 50 cycles, the discharge capacity retention of its half-cells is 75% while the FEC-based electrolyte additive is much lower (37%). We have further investigated fresh and cycled electrode surfaces by SEM and XPS techniques and attributed improvements in the cycling performance to changes in the electrode morphology. In addition, the formation of a stable solid electrolyte interfase (SEI) layer that is rich in inorganic species (LiF) and poor in organic compounds leads to enhanced protection of the Si-C electrodes from degradation. This work opens the door to a different class of battery additives with the potential to enhance the energy density of Li-ion batteries.